It is known that natural starch found in vegetable products that contains a defined amount of water can be treated at an elevated temperature and in a closed volume, thereby at elevated pressures, to form a melt. The process is conveniently carried out in an injection molding machine or extruder. The starch is fed through the hopper onto a rotating, reciprocating screw. The feed material moves along the screw towards the tip. During this process, the temperature of the material is increased by means of external heaters around the outside of the barrel and by the shearing action of the screw. Starting in the feed zone and continuing in the compression zone, the particulate feed becomes gradually molten. It is then conveyed through the metering zone, where homogenization of the melt occurs, and then to the end of the screw. The molten material at the tip can then be treated further by injection molding or extrusion or any other known technique to treat thermoplastic melts, to obtain shaped articles.
This treatment, which is described in U.S. Pat. No. 4,673,438, which patent is incorporated herein by reference, yields a substantially destructurized starch. As described in the above-mentioned patent, the reason for the destructurizing is that the starch is heated above the glass transition and the melting temperatures of its components. As a consequence, a melting and disordering of the molecular structure of the starch granules takes place, so that a substantially destructurized starch is obtained. The expression "destructurized starch" defines starch obtained by such thermoplastic melt formation. Reference is also made to U.S. patent application Ser. No. 209,151, filed June 20, 1988, now U.S. Pat. No. 4,900,361, Ser. No. 209,402, filed June 20, 1988 and now abandoned and Ser. No. 278,116, filed Nov. 30, 1988 and now abandoned, which further describe destructurized starch, methods for making it, and uses of it. These applications are also incorporated herein by reference.
It is preferred that the destructurized starch used in the present invention has been heated to a high enough temperature and for a time long enough so that the specific endothermic transition analysis as represented by a differential scanning calorimetry (DSC) curve indicates that a specific relatively narrow peak just prior to oxidative and thermal degradation has disappeared, as described in the above-mentioned application Ser. No. 278,116.
Destructurized starch is a new and useful material for many applications. An important property is its biodegradability. In humid air, however, destructurized starch takes up water from the air, thereby increasing its moisture content. As a consequence, a shaped article made from destructurized starch may lose its dimensional stability under such conditions. On the other hand, such an article may dry out in low humidity and become brittle.
Thermoplastic starch has a unique set of properties and, while these properties make destructurized starch very useful, these same properties may limit the utility of destructurized starch in cases where a softer, more resilient or a harder, tougher polymer is desired.
Thermoplastic starch, as mentioned, can be extruded and molded into numerous useful shapes, profiles and products. However, the processing parameters such as water content, temperature, and pressure must be narrowly controlled to achieve reproducible quality products. This is a further disadvantage for many commercial applications.
To overcome these potential limitations, it would be useful to increase the dimensional stability over a wide humidity range; to increase the toughness (measured as break energy); to increase the elasticity (measured as elongation); to decrease polymer stiffness (measured as Young's modulus) and to increase the hardness.
By broadening the processing latitude it is possible to increase the variety of shapes and composites that can be made on a commercial basis and to decrease the need for close controls. It would therefore also be useful to improve the control of the melt strength, e.g. increasing the processing latitude for extruding, injection molding, film blowing or fiber drawing and to control surface tack and adhesion to other substrates.
Conventional thermoplastic materials are hydrophobic, substantially water-insoluble polymers that are conventionally processed in the absence of water and volatile materials. Starch, on the other hand, forms a melt in the presence of water but decomposes at elevated temperature. i.e. around 240.degree. C. Therefore, it was expected, in view of its hydrophilic nature and chemical structure that such a starch melt would not be useful as a thermoplastic component together with hydrophobic, substantially water-insoluble polymeric materials.
It has now been found that starch, when heated in a closed volume at proper moisture and temperature conditions as described above to form a melt of destructurized starch, is substantially compatible in its processing with melts formed by hydrophobic substantially water-insoluble thermoplastic polymers and that the two types of molten materials show an interesting combination of properties, especially after the melt has solidified.
One very important aspect is the surprisingly improved dimensional stability of such destructurized starch blended with such hydrophobic thermoplastic materials. For example, by blending destructurized starch with merely 1% by weight of a thermoplastic synthetic polymer (component (c)), without the use of component (b), a shrinkage of less than 4% is observed after two days in shaped articles such as long, narrow rods. This is to be compared to a shrinkage of up to 40% in length when such articles are comprised only of destructurized starch and are exposed to humidity in the air. Shrinkage in these instances may occur within a few hours. Such polymer compositions are described in copending U.S. patent application Ser. No. 298,603, filed Jan. 18, 1989.
Although articles made from such compositions, components (a) and (c) admixed, possess better dimensional stability than those made from destructurized starch alone (component (a)), the physical properties of the therein-described compositions, while useful for certain important applications, are not as good as might be desired for other end uses. In particular, it is important that articles made from destructurized starch compositions retain sufficient strength and dimensional stability to perform their desired functions while still being biodegradable after disposal.
It has now been found that articles made from such destructurized starch blended with specific hydrophobic thermoplastic materials as described herein show a surprising increase in all or a part of their physical properties and behavior of their melts and overcome the limitations explained above. Moreover, and surprisingly, many of the blends described herein show a significantly improved dimensional stability in humid air compared with non-blended destructurized starch while retaining a surprisingly high degree of disintegration in contact with liquid water, which in consequence leads to a high degree of biodegradability.